Together with a recent tendency of higher degree of integration density of semiconductor devices, there has been an increasing demand for a higher-speed operation. In an optical communication system, for example, data transfer rate has been significantly increased and data transfer rates of 2.4 Gbps and 10 Gbps have already been realized in practical use. In the future, the data transfer rate is expected to be increased more and more.
As the operational speed of the semiconductor device becomes higher, it is impossible to ignore the presence of a reflected signal generated in a transmission path that connects together interconnections within a semiconductor integrated circuit (IC) or connects the semiconductor IC with another semiconductor IC. Therefore, a technique of impedance matching is used in a portion having a long interconnection or long transmission path.
FIG. 9 shows the configuration of a module in a conventional semiconductor device. A semiconductor device 21 has input/output connectors 27 connected to an external circuit. Each terminal of the connector 27 is connected to a corresponding input/output transmission path 22. Disposed on a mounting substrate/board 28 are a plurality of cascaded IC chips. More specifically, an input-side IC chip 24 and an output-side IC chip 26, which are connected to the input/output transmission paths 22, and the intermediate IC chips 25 are disposed on the mounting substrate/board 28. Each adjacent two of the IC chips disposed are connected together by one or more internal transmission paths 23 formed on the mounting substrate/board 28. Each of the connectors 27 is connected to a coaxial cable (not shown), through which a signal is exchanged between the semiconductor device 21 and the external circuit. Here, a propagation in the coaxial transmission path will be described. Attenuation constant α is represented by the following equation:
                    α        =                                            1                              4                ⁢                                                                  ⁢                π                                      ⁢                          Rs                                                                                          μ                      0                                        ɛ                                    ⁢                  ln                  ⁢                                      D                    d                                                                                =                      (                                          1                d                            +                              1                D                                      )                                              (        1        )            where d is the outer diameter of a central conductor, D is the inner diameter of an outside conductor, ε is the dielectric constant of insulator, Rs is the surface resistance, and μ0 is the magnetic permeability of vacuum.
A smaller value of the α reduces the attenuation in the coaxial transmission path. In the above equation (1), from the view point of the relation between the ratio of D to d and α, if D/d=3.59, then α assumes a minimum value. The characteristic impedance of the coaxial cable is represented by the following equation:
                              Z          0                =                              60                                          ɛ                r                                              ⁢          ln          ⁢                      D            d                                              (        2        )            where εr is the relative dielectric constant of insulator.
The coaxial cable uses polyethylene resin (εr=2.3) as an insulator to support the central conductor. When this relative dielectric constant is assigned to the coaxial cable using the support for the central conductor, the characteristic impedance is about 50 Ω (ohms) at the point of D/d=3.59, where the attenuation constant assumes the minimum value. Therefore, the external impedance is generally set at 50 ohms. The characteristic impedance of the input/output transmission path 22 is accordingly set at 50 ohms in order to achieve the impedance matching to the external impedance. At the same time, the characteristic impedance of the internal transmission path 23 is also set at 50 ohms. The input/output impedance of the ICs (24 to 26) that perform a high-speed operation is also set at 50 ohms.
In the conventional technique, in order to set the input/output impedance of IC at 50 ohms, the input circuit of the IC is connected to a matching resistor of 50 ohms and, at the same time, the output circuit of the IC is also connected to another matching resistor of 50 ohms. Therefore, the output transistor of the IC is driven by a low load resistance. In the IC to which an output signal from the output transistor is input, the amplitude of the input signal is defined in order to guarantee a normal operation of the IC on the signal input side. The output transistor on the preceding-stage IC operates to meet the defined amplitude. However, being connected to a low load resistance as described above, the output transistor needs to be driven with a large current in order to meet the requirement. As a result, the following problems have arisen:    (1) The current dissipation of the ICs (24 to 26) and semiconductor device (21) is increased because the output transistor is driven with a large current.    (2) The size of the IC should be increased because the size of the output transistor needs to be increased.    (3) The current dissipation and the area of the IC are increased more and more because the number of stages or the size of the buffer circuit for driving the output transistor is increased.